1. Field of the Invention
The present invention relates to an exhaust emission control system for an engine that is mounted on a vehicle such as a motorcycle.
2. Description of the Related Art
Japanese Patent Laid-open No. 2005-299529 discloses a technology for contriving early activation of a catalyst, wherein an on-off valve for making variable a passage cross-sectional area is provided immediately upstream of the catalyst, and the on-off valve is closed during cold-engine operation so that an exhaust gas flows to the catalyst in a concentrated manner through a small-diameter passage oriented toward the center of the catalyst, thereby promoting a rise in the temperature of the catalyst.
In the technology disclosed in the above-mentioned Japanese Patent, in order to activate the catalyst earlier, a configuration may be contemplated in which the exhaust gas at a higher temperature is led to the catalyst by, for example, disposing the catalyst closer to an exhaust port. In that case, however, the exhaust gas at a higher temperature will flow to the catalyst during high-load operation, which is undesirable from the viewpoint of protection of the catalyst.